Proteins are central to all living processes (Figure 11.1). Many proteins are enzymes, the biological catalysts that drive the chemical reactions of the cell; others are structural components, providing scaffolding and support for membranes, filaments, bone, and hair. Some proteins help transport substances; others have a regulatory, communication, or defense function.

AMINO ACIDS All proteins are polymers composed of amino acids, linked end to end. Twenty common amino acids are found in proteins (Table 11.1). All of these amino acids are similar in structure: each consists of a central carbon atom bonded to an amino group, a hydrogen atom, a carboxyl group, and an R (radical) group that differs for each amino acid (Figure 11.2a). The amino acids in proteins are joined together by peptide bonds (Figure 11.2b) to form polypeptide chains; a protein consists of one or more polypeptide chains. Like nucleic acids, polypeptides have polarity: one end has a free amino group (NH₃⁺) and the other end has a free carboxyl group (COO⁻). Some proteins consist of only a few amino acids, whereas others may have thousands.

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PROTEIN STRUCTURE Like that of nucleic acids, the molecular structure of proteins has several levels of organization. The primary structure of a protein is its sequence of amino acids (Figure 11.3a). Through interactions between neighboring amino acids, a polypeptide chain folds and twists into a secondary structure (Figure 11.3b). Two common secondary structures found in proteins are the beta (β) pleated sheet and the alpha (α) helix. Secondary structures interact and fold further to form a tertiary structure (Figure 11.3c), which is the overall, three-dimensional shape of the protein. The secondary and tertiary structures of a protein are ultimately determined by the primary structure—the amino acid sequence—of the protein. Finally, some proteins consist of two or more polypeptide chains that associate to produce a quaternary structure (Figure 11.3d).